Similarity of nearly everyone in agony suffering (7)
I believe the answer is:
analogy
'similarity' is the definition.
(analogy is a kind of similarity)
'nearly everyone in agony suffering' is the wordplay.
'nearly' means to remove the last letter.
'everyone' becomes 'all' (all people).
'in' is an insertion indicator.
'suffering' is an anagram indicator.
'all' with its final letter taken away is 'al'.
'agony' is an anagram of 'anogy'.
'al' put within 'anogy' is 'ANALOGY'.
'of' is the link.
(Other definitions for analogy that I've seen before include "A comparison or parallel" , "Comparison made to aid clarification" , "Likeness or similarity in some ways" , "Partial likeness between two things being compared" , "Helpful comparison" .)